Design an object-oriented, in-memory publish-subscribe model. Requirements: - A publisher can publish a message to a topic. - A subscriber can subscribe to or unsubscribe from a topic. - When a message is published to a topic, all current subscribers of that topic should receive the message. - The design should support multiple topics and multiple subscribers. - Define the main classes, interfaces, and public APIs. - Explain the delivery behavior, thread-safety considerations, and how you would extend the design for retries, persistence, or asynchronous delivery.